Method of heat treatment.



W. l. WRIGHTON.

METHOD 0F HEAT TREATMENT.

APPLlcATmN FILED JUNE 15. 1915.

l 1 88, 1 28. Patented J ung 20, 1916.

n A JI lf R1 RZ" 3- 3A@ B' 11:/ Ffa- 7 7 4 T/ .s q L s Maj# METHOD OF HEAT TREATMENT.

Speoioation of Letters Patent.

Patented June 20, 1916.

Application led June 15, 1915. Serial No. 34,249.

pensed with; are gained, the method depending upon utilization of the occurrence simultaneously within the material under treatment and external thereto of a change or changes in the rate of change of temperature Within the material and a change o1 changes in the or changes in the perature or other condition external to the material being determined by any suitable device responsive to heat energy, radiant energy, or both, the method being further independent of the relation of the magnitude of the temperature Within the material to the magnitude of the temperature or other condition external to the` material, the method being further independent of direction of flow of heat or radiant energy to or from the material under treatment.

My invention resides also in a system or apparatus for carrying out my improved method.

For an understanding of my improvement and for an illustration of one mode of practising the same, reference may be had to the accompanying drawings, in which:

` l is a graphiclrepresentation of particularly referred methods of accom- Fig. 2 is a l under treatment to points Fig. 3 is a graphic representation of the relation between temperature and time and constituting a typical curve utilizable for the practice of my improvements, the rate of heating being slower than the rate of heating illustrated in Fig. l. Fig. 4 is a diagrammatic illustration of treatment.

It has heretofore been the common practice in heat treating steel, as in the hardening of steel, to endeavor to heat the steel to or beyond the decalescence stage or a critical point by raising the temperature of the surrounding medium to a predetermined value and then to quench or suddenly cool the steel by plunging it into water, brine or other cooling medium with resultant hardenin of the steel, the hardness being thereafter reduced to any suitable or desired degree according to the purpose in mind, by well known processes.

In the old method it is customary to determine the temperature to which the steel must be brought to insure reaching or assing beyond the decalescence stage or critical point by experimentally determining such temperature from one or more small samples of the steel to be treated. Then the mass of steel to betreated is placed in a furnace and raised in temperature until a py rometer adjacent thereto or in contact therewith indicates the temperature to which it is desired to bring the steel under treatment as previously determined by experiment upon a sample as stated above. The heating is then continued while maintainingthe furnace at such temperature, for a period of time depending upon the mass of steel to be treated, a longer time for a greater mass, such period of additional heating being termed the soaking period which was supposed necessary to insure the desired result. This method of heat treating steel is at fault in several respects and the results attained thereby so various and uncertain as to make it difficult for even a skilled artisan to reproduce results.

Among the errors of the usual methods, some are the following: 1. The sample from which a preliminary test is made may not be of the same composition or characteristics as the steel actually to be treated. 2. The sample may have its critical point wrongly determined or it may actually be changed by undue or prolonged heating. 3. The pyrometer used to determine the temperature of the steel under treatment may be in error. 4. The pyrometer at best merely indicates its own temperature which may be and usually is different from that of the steel being treated. 5. r1`he rate of imparting heat to the steel is by no means to be determined by the actual pyrometer readings, and as will be shown, this rate is importan A reference to Fig. 2 shows that there is almost a certainty that with the usual method of employing pyrometers, the pyrometer will be at a temperature higher than that of the steel being treated. With a gas furnace, the hot gases impinging upon the pyrometer will usually bring its temperature up faster than that of the steel. With an electric furnace, where the energy is introduced by means of a resistor of wire or other material, the same condition will subsist only to a lesser degree unless the furnace temperature builds up with the temperature of the steel at a rate slow enough to permit the steel to gain in temperature practically as rapidly as the furnace.

In Fig. 2 the point g represents the temperature at a point external to the mass t0 be treated. The point h represents the temperature at a point immediately within the outer surface or skin of the mass to be treated; and the point e' represents the temperature at a point at substantially the center of the mass. This figure indicates therefore, by way of illustration merely, that there is a relatively sharp fall of temperature or relatively sharp temperature gradient between a point external to the mass and a point just within the surface of the mass, and that from a point just within the surface of the mass the fall of temperature or temperature gradient is small, substantially zero, throughout the interior of the mass.

In making determinations of temperature within a mass of steel, even in the case where the mass of steel was 12 inches in `diameter and Q feet long, at different distances from the surface,and simultaneously determining the temperature near the outer surface of the steel, I have found that there is a relatively great difference in temperature between all points within the mass of steel and points outside of the steel but adjacent thereto, whereas among all points within the steel there was always but a small difference and furthermore this small difference was always in the nature of a relatively small temperature gradient from the heated surfaces to points more remote. In other words, owing to the slight resistance which steels offers to the flow of heat, there is a small fall of temperature from the outer surface where it is receiving heat to more remote points. Of course, even this small temperature gradient ceases to exist when the steel is at the same temperature as its surroundings. Furthermore, when a mass of metal is receiving heat from a surrounding medium and consequently rising in temperature, there is a very considerable temperature gradient throughout that portion of the surrounding medium which is in close proximity to the steel or mass of metal being heated. As a consequence any thernio-couple or similar pyrometer located in proximity to the heated mass is receiving some of its heat by radiation from the heated masses and from the walls of the furnace and some by convection and conduction from the gaseous surrounding medium. Hence a pyrometer so located offers no certain criterion of the temperature of the mass being heated. And for a similar reason a radiation pyrometer may give an indication depending more upon the hotter gases surrounding the metal heated than upon the metal itself.

Various tests have been made in support of the statement that there is very little temperature difference within a mass of metal being heated. In making such determinations, I proceeded as indicated in Fig. 4 where E represents, in section, the furnace or lire chamber of an electric or fuel heated furnace and S represents the steel mass.

At different distances from the surface,

and within the mass, S, were disposed a plurality of thermo-junctions, T1, T2, two only being here represented, though I have used a greater number simultaneously with no difference in the result. Within the atmosphere or the hot gases of the furnace adjacent the mass S or even in contact therewith,l was placed a thermo-junction T3. Associated with the thermo-junctions were the recorders R1, R2 and R3 respectively, of Well known construction, for drawing by their pens or markers 1, 2, 3 upon record sheets such as illustrated in Fig. 1, continuous curves representing in the well known manner the relations between temperature and time. Each of the recorders and associated thermo-couple was previously carefully calibrated so that the resultant curves would give the true temperatures at the respective thermo-junctions.

Referring now to Fig. 1, which represents the various record sheets from the three recorders of Fig. 4, it will be found that while the curve A indicating the temperature changes of the thermo-couple T3 is quite different from that of T1 or T2, the last two are so nearly identical that the may be shown as a single curve B since with the scale to which they are drawn they are hardly distinguishable. The curve A, then, is the time-temperature curve produced by the recorder Rs associated with the thermojunction T3, in contact with or near the surface, s, of the mass of steel. Ordinates or vertical distances measured parallel to the axis O-Y represent temperatures, while absciss or distances measured parallel to the axis O`X represent time. From the point a to the point b on the curve A the mass of steel S was receiving and absorbing heat and rising in temperature; from the point Z) to the point c of the curve the mass of steel was cooling or giving out heat. Simultaneously with the making of the curve Ra the recorders 1 and R2 were producing curves so closely similar and co-incident that they are represented in Fig. 1 as a single B. This identity of curves from T1 and T2 shows that although the mass of steel may be at a temperature quite different from the surrounding medium, the various points of the mass itself Will differ but slightly in temperature among themselves, such small differences being accounted for by the heat resistivity of the steel or mass of metal and existing because of and during the flow of heat inwardly from the outer surface while the temperature is rising and outwardly While it is falling. From the point d to the point e on curve B the temperature within the mass S increased and from point e to point f the temperature decreased.l

Aside from demonstrating then the fact that there is a practical uniformity of temperature throughout the mass being treated, there is also demonstrated the further fact that there is a relatively considerable difference of temperature as recorded by a thermocouple or other device Within the mass of by the recorder metal and as shown by a similar device outside of the mass of metal being treated, even though the latter device be in contact with the outer surface of the metal S. Of course, the magnitude of these differences in temperature at any given instant represented by the differences of the ordinates of the curves A and B depend upon the type of furnace of tiring. If fired so ifl'erence as indicated between curves A and B will be relatively small. While this condition may be approximated with an electric furnace of the resistor type, it is dificult of attainment with a gas heated furnace, consequently it is not unusual to find a marked difference between the indications of a temperature measuring device outside of a mass of metal and the actual temperature of the mass under treatment.

ne reason for the common practice of soaking is now evident. If the furnace temperature is kept at the desired high value for a long enough time, the mass of steel will also arrive at that temperature.4 But since it lags behind the furnace temperature an amount depending upon various conditions such as the method or rate of firing it is evident that it will not be brought up to the temperature of an outside indicating device excepting after a considerable length of time (theoretically after an infinite time). Therefore to insure final arrival at a temperature desired but not passing beyond that temperature it has been customary to hold the furnace at the desired temperature for such a length of time as experience has indicated is necessary to properly heat treat the steel. It has been a mere inference that the steel has itself arrived at the desired temperature, this conclusion being in part reached by noting the nature of the resulting product and in part from the knowledge that the steel will eventually arrive at the temperature of the surrounding medium if said medium is held long enough at one temperature. It may take several hours for a piece of steel to attain the temperature of the furnace and moreover this time is influenced by the method or rate of tiring the furnace and the mass and shape of the piece being treated. Consequently it is clear that not only is a vast amount of experience demanded when attempting to judge of the temperature of a mass of metal so heated, but minor circumstances are continually preventing a good judgment of the actual conditions such, for example, as the influence of other masses that may happen to be heated at the same time or the relative position of the various masses with respect to the furnace walls or entering gases, if a gas furnace is employed. Another reason for soaking is based upon the assumption that time enters as a necessary element into the completion of a certain chemical and physic'al transformation. But here also, unless such soaking is performed at a proper and known temperature, no certain conclusion can be drawn as to the sufficiency of the operation.

To restate briefly, then, the objection to reliance upon a temperature indication as a mode of heat treatment: If a pyrometer be employed adjacent to the steel to determine the steel temperature, it will generally show a higher temperature than the actual ternperature of the steel. Hence if the steel were removed at once it would be at too low a temperature. If, however, it were allowed to remain until the entire furnace were in heat equilibrium, then the external pyrometer would be at the same temperature as the steel and consequently would be a safe guide as to the steel temperature. But even when this soaking process is resorted to the best results are not obtained because aside from the waste of time and unnecessary expense of heat energy that result, such prolonged heating of steel is not conducive to the best product, some desirable qualities being lost from such prolonged heating. It is evident that this long time of heating, in order that an outside ternperature indicating device may correctly measure the steels condition is a process that necessarily occupies much time for when the steel approaches the temperature of the furnace the process of heat transfer is necessarily slow.

A very important feature which is brought out by the curves of Fig. l, and upon which my improved method largely rests lies in the fact that although the external energy responsive device which produced the curve A shows an actual temperature at all times (during the increase of temperature of mass S) higher than the temperature within the mass S, as recorded by curve B, still curve A shows certain inflections namely at j, la, q, r, which will now be considered and which if properly understood oder a precise means of determining the condition' of mass S as to temperature or passage through critical points.

Referring first to curve B, which may be taken to represent the time-temperature relation at any point within the mass of steel S being treated, we notice that initially the increase of temperature of mass S with time is fairly uniform and represented by a line which is nearly straight but which, however, makes an angle with the base whose magnitude is dependent upon the rate at which the furnace is fired if it is being heated up or upon the relative heat capacity of the mass S and furnace or both. It is evident then that while this line from d to some point o the decalescence period may slant more or less, it may be held fairly straight by suitable firing. When, however, the true temperature of the steel has attained a certain value dependent somewhat upon the nature of the steel the rate at which the temperature of the steel rises becomes less and this change is sufficiently abrupt to cause the time-temperature curve to assume a new slant sufficiently differing from the last slant to .be noticeable on the record. During this period the steel is passing through the decalescence stage or critical point and is absorbing heat without a corresponding rise of temperature. This well known phenomenon is similar to that occurring when ice is transformed into water or water into steam. In other words: From the point cl to the point o on curve B the rise of temperature within the metal is quite regular and more or less uniform, but when the temperature yo has been reached, there is a change in the rate of change of temperature which extends for a periodof time until the point p is reached, when there is another change' in the rate of change of temperature, and from point p to the point e the rate of change in the temperature becomes more or less equal to that subsisting between d and o depending upon the firing of the furnace.

The time represented by the part of the curve between 0- and p is characterized as or stage, during which, notwithstanding continued delivery of heat to the steel at the same or substantially the same rate as between d and o, the mass does not rise in temperature as rapidly as before. In other words, during the decalescence stage the mass keeps on absorbing heat, but increases in temperature at a lower rate. This is generally understood to be the result of work done or energy expended upon the material of the mass itself in creating a change therein as by causing carbon of the steel which, up to the temperature o was merely in physical mixture with the ferric material, to go into solution in such ferrie material, the act of dissolving requiring expenditure of energy, represented by heat delivered to the mass; and because of such absorption of energy for producing the solution the temperature does not rise at the rate at which it rises before solution begins. At the point p, the solution may be considered completed and thereafter the mass again rises in temperature at substantially uniform rate and proportionately to the increase in heat units absorbed thereby. As stated, this decalescence stage is well known; and in the old method of heat treatment it was common, as far as concerned hardening of steel, to attempt to carry the steel through the decalescence stage, that is, to some point beyond the point p, as, for example, to the point w, an

d lso then to quench or suddenly cool it. But by the old method it was assumed that if a predetermined temperature, determined by experiment with a small sample, were indicated on a responsive device at or near the surface of the mass, the mass had passed through the decalescence stage and was ready for quenching. I have found, however, as illustrated in Fig. 1, that the reaching or passing beyond the decalescence stage, Y

so far as concerns all of the mass of the steel, is independent of the actual temperature existing outside of thel mass, and that, as a matter of fact, the mass passes through such decalescence stage or critical point at a temperature considerably lower than the temperature existing at the same time outside of the mass of metal, during heating at a rate above a predetermlned minimum.

An inspection of curve A will show that although the temperature being recorded by this curve is not that of the steel being treated, still there are shown on this curve inflections similar to those of the mass treated and moreover, while these inflections occur on the curve A at temperatures higher than on curve B they occur at the same time. This very important fact means that although the device recording the curve A may not be used as a means of determining the temperature of the steel, it may be used for determining the time of occurrence of the decalescence stage or its passage. Moreover this is really what is required in heat treating steel. The actual temperature is notrequired, but it is necessary to know when the steel has assed the critical point. The curve A re ects, so to speak, the temperature occurrences in the steel as represented by the curve B and consequently the necessity does not arise for determining the form of the curve B, but only of the curve A. It is evident, moreover, that since the curve A does not indicate the actual temperatures of the steel it is not necessary for the responsive device producing the curve A to be correctly calibrated. An apploximate calibration of it, however, might prove at times desirable. Snpposing, for example, experience has shown that the particular brand of steel to be treated undergoes best treatment by carrying it 100 degrees above the critical point p. If now we carry the temperature of the device producing the curve A 100 degrees above 7c we know if this device is approximately calibrated that we have carried the steel substantially 100 degrees above p. The responsive device may have actually an error in its indications whereby for example all of its readings are say 50 degrees low or high, but this does not prevent getting a measure of the 100 degrees difference as between the point lc and some point as n above k. The reason for these inflections j and Il: inthe curve A occurring at the same time with in- `lections o and fp in the curve B is that the uniform rise o temperature of the responsive device is interfered with by the rather sudden change 1n the uniform rise of temmass of steel as it reaches or passes beyond the decalescence stage. the steel stops its former rate of temperature increase it stops sending out heat by radiation or conduction at the former rate and the temperature gradient without the steel changes. Consequently the responsive device situated nearby or in contact therewith receives heat at a lessened rate. It therefore records a change in the shape of curve just as though it had gone through a decalescence stage at the same time as the steel but at a diiferent temperature. In other words, the fact upon which my improved method is largely based is that the decalescence period of the interior of a mass being heated begins and ends simultaneously with a similar temperature change occurring in a heat responsive device located adjacent the mass, and that the temperatures at the beginning and the end of the decalescence period Within the mass are different from the corresponding temperatures at the same moments on the outside of the mass as indicated by an energy responsive device external thereto.

The decalescence period 0-p of curve B is the true decalescence stage of the mass of steel and is determinative of the critical points which must be reached or passed for various predetermined treatments. The period j-la of curve A is a reflection as to time of the curve stage o-p and since stage j-lc occurs practically at the same time as stage o-p the true critical points Within the mass are accurately determined as to time and independently of temperature from the curve A which is a relation between temperature (which need not be known) and time for a material external to the mass treated. Therefore, for the practice of my method I need not determine the actual conditions within the mass under treatment, but may produce a curve A, as by using only one of the recorders, as R3 in conjunction with a thermo-junction or resistance pyrometer "13 external to the mass, which curve A will show when the critical points or decalescence stage occur.

While in this description I have referred specifically to a thermo-couple as a means of determining the shape of curve A I do not limit myself to the use of such a device, but any energy responsive device may be used which is competent either to control production of such curve or furnish such data as will enable such curve to be plotted or simply visualized. The energy responsive device may be responsive to heat energy, radiant energ or both, and it may automatically record the chan es in the magnltudes to which it respon s or may merely indicate such changes by the progressive motion of a pointer, for example, or a spot of light. And furthermore, the energy responsive device may be one which requires manual adjustment to accommodate it to such changes in magnitude. For illustration, a thermo-couple may be employed which is responsive to radiant energy and energy in the form of heat delivered by conduction or convection. The steel being treated may be heated electrically in a vacuum and the responsive device will then not only receive its energy by radiation but such device will be at a temperature lower instead of higher than the steel. The curve A will then lie below B where it ascends but the iniiection points 7' an 1c will still agree in time with the points o and p on curve B. Again a resistance thermometer may be utilized which is also responsive to both forms of energy. Or a radiation pyrometer of the resistance' or thermo-couple type may be employed, in which case they would of course be located Without the furnace Walls.' Or a radiation pyrometer may be employed which is responsive to those radiations lying Within the visible spectrum. In this case the usual match in color or brightness is attained by hand adjustment and the change in the rate of change of this adjustment may serve to indicate when the decalescence period or any critical point is v reached or passed. 1t is evident that other forms of responsive device might be employed that are responsive to the changes 1n the flow of heat energy or radiant energy to or from the steel being heated.

As illustrated in Fig. 5 the thermo-junction for producing curve A may be placed very close to or in contact with the surface of the mass S as indicated at 4, or it may be placed as at 5 a substantial distance from the mass S and at a substantial distance from the inner Wall of the furnace F; or as indicated at 6 it may be placed at `or near the inner wall of the furnace F. I have found that wherever the thermo-j unction is placed outside of the mass S,the general relation between curves A and B as to simultaneous occurrence of the decalescence period or critical ,points obtains. That is, during the decalescence period o, p of the material of the mass S there exists a similar and simultaneous change in the rate of temperature change in the heat responsive device at or near the surface of the mass S. Accordingly by my improved method the thermojunction or other heat responsive device may be placed in contact with or near the mass S and even at a considerable distance therefrom and such thermo-junction or other responsive device may control a recrder as R or other curve drawing instrument to produce a curve such as A, Fig. l, or Fig. 3. The instrument in such a case need not be calibrated in temperature, and in fact the true or absolute temperature may be entirely ignored because, as shown in Fig. 1, the beginning and end of the decalescence stage is indicated by the energy responsive device outside of the mass S The instrument draws a curve beginning, for example, at the point a and continues to the point j, the beginning of the decalescence period Within the mass, the point j being simultaneous with the point o, then there occurs a change in the rate of temperature change within the mass which is reected by a change in the rate of temperature change in the heat or radiant energy responsive device as indicated by the part of the curve from j to 7:. Thereafter the curve continues on beyond the turn or second hump la and When such point c has been reached or passed, as may be desired by the operator, the mass may be quenched or carried through Whatever subsequent treatment is necessary with the assurance that the interior of the mass itself has previously reached or passed through the decalescence stage, though the temperature both Within the mass and outside of it are not or need not be known.

By this method of noting a change in the rate of temperature change outside the mass under treatment and without regard to the true or absolute temperature involved, the beginning of, end of, or passage beyond the decalescence period of the interior of the mass is reflected by the condition of the heat responsive device external to the mass; and the resulting advantages over the old method are, nter alia, the following:

(a) A mass of steel may be immediately put into the furnace and heated to the desired stage without preyiously testing or making the temperature determinations on a sample, because my method above described is independent of true or absolute temperatures. (b) The soaking of the mass, according to the old method, may be dispensed with because, as shown by curve B, all parts of the mass pass through the decalescence stage at a time indi` cated by the heat responsive device irrespective of the temperature of the latter. There is therefore no occasion for continued heating in order to insure carrying through the critical temperature. By dispensing with the soaking period fuel or heating energy and time are greatly economized and to that extent the cost of heat treatment is reduced. (c) The method being independent of true or absolute temperatures involved avoids all possibilities of error of the old method, due to errors in standardization or calibration of a temperature measuring device. (d) Certainty and uniformity of results unattainable by the old method which relied upon true or absolute temperatures supposed to determine conditions with respect to the decalescence stage. (e) The use of a thermo-junction or other heat responsive device which with its associated curve drawing instrument need not be calibrated, because true or absolute temperatures need not be known or determined. (f) Supplanting chance, approximations, and effects of variables of composition of old method by certainty, accuracy and independence of composition.

When the mass S is allowed to cool it is at higher temperature than its surroundings, in order that cooling may occur and this higher temperature is indicated from the fact that curve B, Fig. 1, is above curve A to the right of the maximum.

It is well known that in cooling, the mass passes through a recalescence stage during which, while actually losing heat, its temperature actually increases, or in any event decreases at a slower rate. This is indicated in Fig. 1. The beginning of the period is indicated at g and the end at r on curve B. The beginning and end of such recalescence stage is also reflected in the heat responsive device outside of the mass S as indicated by the curve A where the point t indicates the beginning and u the end of such recalescence period. Here again as in the case of the decalescence period, the beginning and end of the phenomena of recalescence occurs Simultaneously with the change in the rate of temperature change of the heat responsive device and accordingly the latter indications are a true criterion of the condition of the steel and its time of passage through the recalescence period. It will therefore be understood that my method is also available for determining, without regard to true or absolute temperatures, the beginning and end of the recalescence stage within the mass of material by observing change of heatconditions external to the mass.

In Fig. 3 is illustrated a curve A such as may be obtained by the practice 'of my method in which, however, the rate of heating is slower than in the case of Fig. 1. Thus the slopes of the curve in Fig. 3 between the points a and j and between c and Y) are not so great as the slopes between the corresponding points in Fig. 1. Because of this slower rate of heating, that is, slower rate of delivery of heat energy into the mass and into the medium surrounding the mass, the slope between the points and c in Fig. 3 is far less than between the points j and f: in Fig. 1. It will therefore be apparent that my method is applicable to different rates of heating.

It will be understood` that the record sheet for the recorder Ra used in the practice of my method for producing a curve such as A need not be ruled with either time or temperature or other markings, but may be an ordinary blank paper, because, as stated, the temperatures need not be known; and the recorder R3 and its associated thermo-couple or other heat responsive device need not be calibrated.

While I have hereinbefore referred to` the use of a continuous curve drawing instrument for determining the critical points or stages, and such is preferred, it will be understood that any other suitable type of recording instrument may be used, such, for example, as produces a succession of marks or dots which taken together may be considered to outline a curve. And it will be further understood that my method may be practised without recourse to a recording instrument, for the thermo-couple or other energy responsive devicev may be used in connection with any suitable reading instrument and readings taken therefrom at suitable intervals of time, and then such values read from the instrument may be immediately plotted and produce a curve similar to the curve of a recorder which records only separate points on a curve.

It will be understood that my method is not limited to any particular mode ofdetermining the change in a rate of change of temperature in the neighborhood of the mass, nor is it limited to a responsive device sensitive only to heat energy, for as previously shown, a radiation pyrometer whose indications are a function of that radiant energy lying only within the range of wave lengths producing the sensation of light and whose readings are in terms of color, will also permit of operating by my method.

It will be further understood that in place of thermo-junctions or the thermo-couples hereinbefore referred to, a 'resistance pyrometer, or any heat responsive means may be employed. l

Whlle I have hereinbefore referred more particularly to the hardening of steel by carrying it to a( point as 'n on the curve A, and then quenching the same, it will be understood that the determination of change in the rate of change of a temperature refleeting similar simultaneous changes with-.

in a mass or change in the rate of change of the indications of any energy responsive device which rellects any simultaneous changes in a mass undergoing heating or cooling may be used for determining critical points or stages for any other process than the hardening of steel, as for example refining grain, annealing or other treatment of steel or other alloy or metal. For example, for refining the grain of steel, the same may be heated until passage more or less beyond the decalescent stage, that is, to some point as n on the curve A 'beyond the point 7c and then quenched in oil, which causes a quenching or cooling slower than quenching in water or brine used for hardening purposes. And

for annealing steel which has either been previously hardened or Which is in its normal state, the mass is heated until the decalescent stage is begun,but has not ended, and is then slowly cooled as by discontinuing application of heat and the sooner after the commencement of the decalescent stage the cooling begins the better the annealing effect obtained. The dotted curve E, Fig. 1, indicates such cooling taking place in the annealing treatment.

It is further possible by my method of treatment to harden steel, including high carbon steel, Without causing any substantial shrinkage or deformation. This is particularly useful in connection With making punches and dies such as used for punching or forming metal plates, sheets or other forms of material.

A die may be formed with steel in the unhardened state to accurately and nicely iit the punch. It may then be hardened by the practice of my method hereinbefore described, the shrinkage being substantially m'Z, with the result that after hardening the die Will fit the punch With the same accuracy and nicety as before hardening, and Without need for stoning or redressing or grinding the die. This may be accomplished by heating the die, after it has been formed and fitted to the punch, to a point as fu in Fig. 1, beyond the decalescence stage and then quenching when such point is reached The point e to which it is desirable to carry the heating, may be determined for a given material and then all other dies of the same material, and of approximately the same mass, may be similarly treated with the assurance that they Will be suitably hardened Without deformation or shrinkage. In this connection it Will be noted that the point u is on the curve A which reflects a condition Within the mass or die itself, and that the temperature at that time existing either in the mass or 'its surroundings is not or need not be known.

My method is also applicable Where the steel or other metal undertreatment is heated by passing an electric current therethrough to raise its temperature. In such case, while the heat is generated within the mass itself, the heat responsive device may be located close thereto or in Contact therewith, and a curve similar to curve A-can be produced and will reflect the conditions within the electrically heated mass as to the decalescence stage. In such case the temperature external to the electrically heated mass will be lower than the temperature within the mass, but nevertheless the beginning and end of the decalescent period occur simultaneously with the beginning and end of the change in the rate of change of the indications of the energy responsive device external to the mass.

While I have moreparicularly referred to steel having substantia carbon therein, and high carbon steel, it will be understood to be useful for the similar treatment of alloy steels or any other metal, alloy or other material where it is desirable to heat them in a manner related to any critical stages that may make themselves externally manifest.

W'hat I claim is:

1. The method of determining without regard to its temperature when metal, as steel or an alloy thereof, passes through a critical point, which consists in determining at successive times magnitudes related to or dependent upon simultaneous temperatures of a medium external to and in energy exchanging relation with respect to said metal, and noting the time of occurrence of an abrupt change in the rate of change of said magnitudes.

2. The method of determining when metal, as steel or an alloy thereof, passes through a critical point, which consists in determining the time of' occurrence of an abrupt change in the rate of temperature change of a medium external to said metal and in heat exchange relation therewith.

3. The method of determining When metal, as steel or an alloy thereof, passes through a critical point, which consists in determining the time of occurrence of an abrupt change in the rate of temperature change of a medium external to said metal reflecting a simultaneous abrupt change in the rate of temperature change of said metal.

4. The method of determining Without regard to its temperature when metal, as steel or an alloy thereof, passes through a critical point, which consists in causing a change in the heat content of said metal at substantially regular rate, and noting the time of occurrence of an abrupt change in the rate of temperature change of a medium external to the metal reflecting a similar and simultaneous change in the rate of temperature change of said metal.

5. The method of determining Without regard to its temperature when metal, as steel or an alloy thereof, passes through a critical point, which consists in causing the heat content of said metal to changey at substantially regular rate, determining the rate of change of magnitudes differing from but related to temperatures of a medium in energy exchanging relation With said metal, said rate of change of said magnitudes reflecting the rate of temperature change of said metal, and noting the time of occurrence of an abrupt change in said rate of change of said magnitudes.

6. The method of determining Without regard to its temperature when metal, as steel or an alloy thereof, has been heated to a decalescence stage, which consists in causing the heat content of said metal to increaSe at substantially regular rate, noting the rate of temperature change of a medium external to said metal which refiects the rate of temperature change of said metal, and noting the time of occurrence of an abrupt change in said rate of temperature change of said medium.

7. The method of determining without regard to its temperature when metal, as steel or an alloy thereof, has been heated to a decalescence stage, which consists in causing the heat content of said metal to increase at substantially regular rate, determining Without regard to its temperature the rate of temperature change of a medium external to said metal which reflects the rate of temperature change of said metal, and noting the time of occurrence of an abrupt change in said rate of temperature change of said medium.

8. The method of determining without regard to its temperature when` metal, as steel or an alloy thereof, has passed through a decalescence stage, which consists in causing said metal to absorb heat at substantially regular rate, determining the rate of temperature change of a medium external thereto refiecting the rate of temperature change of said metal, and noting the time of.

occurrence of an abrupt increase following an abrupt decrease in the rate of temperature change of said medium.

9. The method of determining when metal, as steel or an alloy thereof, which is changing in temperature passes through a critical point, which consists in making a.`

record in the nature of a time-temperature curve representing the rate of temperature change of a medium external to said metal, and noting the time of occurrence of an abrupt change in the slope of said curve.

10. The method of determining when metal, as steel or an alloy thereof, whose heat content is increasing at a rate which is substantially regular has been heated to the decalescence stage, which consists in making a. record in the nature of a time-temperature curve representingthe rate of temperature change of a medium external to said metal and reflecting the rate of temperature change of said metal, and noting the time of occurrence of an abrupt change in the slope of said curve.

11. The method of determining when metal, as steel or an alloy thereof, whose heat content is increasing at a rate which is substantially regular has been heated through the decalescence stage, which consists in making a record in the nature of a tiine-temperature curve representing the rate of temperature change of a medium external to said metal and reflecting thc rate of tempcrature change of said metal, and noting the time of occurrence of al1 abrupt increase following an abrupt decrease in the slope of said curve.

12. The method of heat treating metal, as steel or an alloy thereof, which consists in causing its heat content to change at a rate which is substantially regular, determining the time `of an abrupt change in the rate of change of its skin temperature, and thereafter cooling it.

13. The method of heat treating metal, as steel or an alloy thereof, which consists in causing its heat content to change at a rate which is substantially regular, determining at successive times magnitudes related to or dependent iipon the simultaneous temperatures of said metal, noting the time of occurrence of an abrupt change in the rate of change of said magnitudes, and thereafter cooling said metal.

14. The method of heat treating metal, as steel or an .alloy thereof, which consists in changing its heat content at a rate which is substantially regular, determining at successive times magnitudes related to or dependent upon simultaneous temperatures of a medium external to and in energy exchanging relation with respect to said metal, noting the time of occurrence of an abrupt change in the rate of change of said magnitudes, and thereafter cooling said metal.

15. The method of heat treating metal, as steel or an alloy thereof, which consists in increasing its heat content at a rate which is substantially regular, determining the rate of temperature change of a medium external thereto which reects the rate of temperature change of said metal, noting the time of occurrence of an abrupt change in said lrate of temperature change of said medium,

and thereafter cooling said metal.

16. The method of hardening steel or an alloy thereof, which consists in increasing its heat content at a rate which is substantially regular, noting the rate of temperature change of a medium external thereto which refiects the rate of temperature change of said steel1 noting the time of occurrence of an increase finllowing an abrupt decrease in said rate of temperature change of said medium, and thereafter' quenching the steel.

17. The method of refining the grain of steel or anv alloyr thereof, which consists in increasing its heat content at a rate which is substantially regular, noting the rate of temperature change of a medium external thereto which retiects the rate of temperature change of said steel, noting the time of occurrence of an increase following an abrupt decrease in said rate of temperature change of said medium, and thereafter slowly quenching the steel.

18. The method of annealing steel or an alloy thereof, which consists in increasing its heat content at a Ver: which is substantially regular, noting the rate of temperature change of a medium external thereto which reiiects the rate of temperature change of said steel, noting the time of oc currence of an abrupt decrease in the rate of temperature change of said medium, and thereafter and before the occurrence of an increase in said rate of temperature change of said medium slowly cooling the steel.

19. The method of treating a mass of steel or an alloy thereof, which consists in increasing its heat content at a rate Which is substantially regular, noting the rate of temperature change of a medium external thereto which reflects the rate of temperature change of said steel, noting the time of occurrence of an abrupt change in said rate of temperature change of said medium, and thereafter cooling said steel without heat soaking the same.

20. The method of hardening a mass of steel or an alloy thereof, which consists in increasing its heat content at a rate which is substantially regular, noting the rate of temperature change of a medium external thereto which reflects the rate of temperature change of said steel, notin the time of occurrence of an increase fo lowing an abrupt decrease in the rate of temperature change of said medium, and thereafter quenching said steel Without heat soaking the same.

21. The method of determining without regard to its temperature when a metal, as steel or an alloy thereof, Wh'ose heat content is changing at a rate which is substantially regular passes through a critical point, which consists in subjecting an energy responsive device to the effects of heat changes occurring in said metal, determining the rate of change of the responses of said energy responsive device, and noting the time of occurrence of an abrupt change in the rate of change of said responses of said energy responsive device.

22. The method of determining Withoutregard to its temperature when a metal, as steel or an alloy thereof, Whose heat content is changing at a rate which is substantially regular passes through a critical point, which consists in determining the rate of change of the effects of ener y exchanged between said metal and a ma external thereto reflecting the rate of temperature change of said metal, and noting the time of occurrence of an abrut change in said rate of change ofsaid e ects.

In testimony whereof I have hereunto affixed my signature this 14 day of J une, 1915.

WILLIAIM J. WRIGHTON.

Which reflects the rate tially regular, noting the rate of temperature. change of a medium external thereto of: temperature change of said steel, noting the time of occurrence of an abrupt .decrease in the rate of temperature cli-ange of said medium, and thereafter and before the occurrence of an increase in said rate of temperature change of said medium slowly cooling the steel.

19. The method of treating a mass of steel or an alloy thereof, Which consists in increasing its heat content yat a rate'which is substantially regular, noting the rate of temperature change of a medium external thereto whichA reflects the rate of temperature change of said steel, noting the time of occurrence of an abrupt change in said rate of temperature change of said medium, and thereafter cooling said steel Without heat soaking the same.

2O. The method of hardening a mass of steel or an alloy thereof,lwhich consists in increasing its heat content-at a rate which is substantially regular, noting lthe rate of temperature change of a medium external thereto which reflects the rate of temperature change of said steel, noting the time of occurrence of an increase followingan abrupt decrease in the rate of temperature change of said medium, and thereafter ,cbrrefioa-lmugfsPatentugr-1,188,128;

'- 1 25th :hay July, A.` 113.3916.

quenching 'sa-.id steel Without heat soaking-- the same. f

21. The method of determining Without i regard to its temperature Whenva metal, as

steel or an `alloy thereof, Wh'ose heat contentl 35 lsponsive de vice to the eects of heat changes A occurring 1n said metal, determining the 40 rate of change of the responses ofv said energyvresponsive device, and noting the time of occurrence of an abrupt change in the 2 rate of change of said responses of said A. v

energy responsive device.

22. yThe method of determining Without regard to its temperature when a metal, as

steel or van alloy thereof, Whose heat content is changing'at a rate which is substantially regular passes through a critical point, which consists in determining the rate of change y I of the effects of ener y exchanged between said metal and a mags external thereto rel'ecting the v rate of temperature change of said metal, and noting the time, of occur- 5b rence of an abrupt change in said' rate .of change ofsaid efects.

In'testimony whereof I have hereunto afxed my signature this 14 day of June, 1915.A

WILLIA/M J. WRIGHTON..

a It shantyarpi im, 'in raap@ No., 1,188,121; granted lnina,2 o,f1"91es,

ilu-pon the applicationlofi ofi Philadelphimg -for an improvement-Methods of lileatu'llreatment," an appears in the Page; linelol, foi: word curvel' should oI'iaY, Aaa am, j am' Correction' In Letters Patent No. 1,188,128-- It is hereby certified that in Letters Patent No. 1,188,128, granted June 20, 1916,

upon the application of Williern J, Wrighton, of Philedelphia, Pennsylvanie, for

an improvement in Methods of Heat. Treatment, an error appears in the printed specification requiring correction as foows: Page 5,` line 101, for. the word curve read true; enti that the said Letter Patent shoulld be' read this correction therein that the same may conform to the record of the oase in the Patent Office.

signeandjmied m8251511 day of Ju1y,A. D., 191e.

F. W. H. CIJAY,

Acting Commissioner qfPam.

[smL] 

